For many years, communication systems have employed direct conversion receivers to process received Radio Frequency (RF) signals. RF signals received by a direct conversion receiver are often converted into an in-phase (I) component and a quadrature (Q) component. As is well known in the art, the direct conversion receiver converts the incoming signal to baseband by mixing it with a Local Oscillator (LO) signal having a frequency that is approximately equal to the carrier frequency of the desired on channel signal.
When the received signal is converted into its constituent baseband I/Q components, I/Q mismatch and intrinsic LO self-mixing may introduce a DC offset error. In direct conversion receiver systems, information modulated onto a received RF signal that is equal in frequency to the LO signal is mixed down to DC voltage within the baseband intermediate frequency (IF) signals. This modulated information, in turn, may be corrupted by intrinsic baseband DC offset errors inherent in stages constituent to the analog baseband signal path including the down conversion mixer, post mixer filtering and gain stages. These errors degrade signal quality. For instance, reverse-transmission paths may occur in a direct conversion receiver that may allow LO energy to couple into the mixer's RF input signal path. As a result, the LO energy at the RF input signal path may self-mix with the LO injected into the mixer and create DC offset errors proportional to the LO coupling into the RF path, thereby affecting signal reception. Thus, detection and correction of DC offset errors is important to improved signal reception.
Various approaches have been attempted to try to avoid distortion of modulated information within direct conversion receiver systems. These conventional approaches generally include using transmit modulation schemes that limit the information proximate to the local oscillator signal, compensating the DC offset error while attempting to maintain the desired baseband DC information using a protocol specific algorithm, or filtering out undesired harmonic distortion artifacts after the received signal is demodulated that may have resulted from inadvertent removal of the desired modulated information during compensation of the undesired DC offset errors within IF signals. Many of these conventional approaches however focus on DC offset correction strategies that are optimized to a specific application or protocol. Often though, signal conditions may change in an operating environment. While a single approach to address DC offset errors may achieve acceptable performance under certain conditions, no single DC offset correction strategy can provide optimal performance for all operating conditions encountered in a portable transceiver. Accordingly, there is a need for improved DC offset error correction under changing signal conditions and circuit operating environments.